Using Modeling to Assess FAC Risk in an HRSG
Flow accelerated corrosion is a highly localized corrosion process that will thin the walls of carbon steel piping that carries water or a two-phase mixture of water and steam in power plant environments. Though the process is gradual, it can result in a pipe rupture if left unchecked.
Industry research has determined that five factors can influence the rate of FAC in a boiler pipe or tube: material composition, fluid temperature, pH, oxygen concentration, and local fluid velocity. Various semi-empirical methods have been developed that use these factors to predict relative wear.
The first four factors can be determined from the design or by direct measurement. The local fluid velocity can be calculated using computational fluid dynamics, yet this factor typically is accounted for by combining the bulk fluid velocity with a "geometry factor" for the pipe section under consideration. FAC can also occur in certain HRSG sections such as the LP evaporator with two-phase mixtures because the velocity of the water component may be very high. Therefore the steam fraction must be taken into account in any analysis.
A boiler model was used to support the FAC risk assessment at a combined-cycle plant with two identical HRSG units. The plant operates in base load with daily operation of the duct burners to augment power production during peak times. The assessment results were in turn used to generate an inspection plan that identified high-risk locations. The assessment used a spreadsheet that listed all the flow circuits.
Bulk flow velocities and temperatures are among the required inputs. These were calculated using steady-state, full-load simulation runs for both fired and unfired conditions.
The results showed that the LP evaporator had a very high circulation ratio in both fired and unfired modes. Therefore, the fluid velocities in the LP evaporator downcomer and inlet and outlet feeders were relatively high. When the appropriate geometry factors were applied to estimate local fluid velocities, the lower feeder pipes to the LP evaporator headers were shown to have the highest risk.
Previous inspections had shown heavy wear at the vortex breaker in the LP drum above the downcomer pipe, which is consistent with excessive flow velocities. Unfortunately, before the inspection plan was implemented, several failures were found at the branches from the downcomer to the lower inlet feeder pipes, in the area predicted to have the highest risk. The unfailed branches all showed considerable thinning. The plant plans to solve the problem by replacing the at-risk sections with a material that is more resistant to FAC (T11 steel with higher chrome content).
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